The present invention relates to a method and a device for determining the course of the bevel at a edge of the spectacle lens to be machined by shape grinding and for controlling shape grinding according to the determined bevel course.
The bevel course along the edge of the spectacle lens to be ground should be determined such that the spectacle lens to be machined can be inserted without further machining steps into a correspondingly selected spectacle frame and, for aesthetic reasons, has along the circumferential edge of the ground spectacle lens the same distance from the front edge, even when the spectacle lens has a shape that deviates greatly from a circular shape, in order to prevent forward projection of the spectacle lens from the spectacle frame by different amounts.
For CNC-controlled grinding, especially grinding of the optical surfaces and the subsequent shape grinding of the circumferential edge and of the bevel of a spectacle lens, the shape of the spectacle lens and the personal optometric data of the patient are known which are supplied in the form of a set of data and with which the spectacle lens grinding machine can be controlled such that the lens blank is machined according to the personal optometric data of the patient relative to optical surfaces as well as relative to the shape of the selected spectacle frame. Also, the bevel (V-shaped bevel) is placed onto the circumferential edge of the ground spectacle lens such that it extends over the entire circumference of the circumferential edge and has, for example, a certain distance to the front edge of the shape-ground spectacle lens.
The data in regard to the shape of the spectacle frame can be present in the form of a data set supplied by the spectacle frame manufacturer or can be determined by inserting the spectacle frame into a device for sensing the bevel groove and by recording the data set in a three-dimensional array. It must be noted in this context that the bevel groove within the spectacle frame is not always positioned on a spherical surface due to manufacturing tolerances or shape deviations.
A device for three-dimensional sensing of spectacle frames is disclosed in German Gebrauchsmuster 86 08 201 U1 of the applicant.
In the published German patent application DE 34 10 040 A1, a CNC-controlled spectacle lens grinding machine is disclosed in which the outer surface and the inner surface of a circular lens blank, clamped into a spectacle lens grinding machine and already finish-machined with regard to optical surfaces, is traced along a curve corresponding to the desired shape of the spectacle lens by two elastically supported, pin-shaped sensors which rest directly on the lens blank. Their axial position is determined by potentiometers whereby the data and values detected by these potentiometers are supplied to a computer for calculating therefrom the spacial curve of the front edge and of the rear edge of the spectacle lens as well as the respective thickness of the spectacle lens. The computer selects from a number of memorized data sets of different V-shaped bevels that particular V-shaped bevel that can be applied to the circumferential edge of the shape-ground spectacle lens without leaving the circumferential edge at any location. The disclosed measuring device is integrated into the CNC-controlled spectacle lens grinding machine, and sensing of the lens blank is always carried out as the first step of the machining process performed by the CNC-controlled spectacle lens grinding machine. During sensing, shape-grinding of the lens blank is not possible.
When measuring a spectacle lens according to German patent DE 38 42 601 C2 of the applicant, whereby the front and rear edges of a pre-ground spectacle lens are measured during the machining process by a sensor head, there is no additional time expenditure during measuring of the spectacle lens. However, deviations of the spectacle lens from the spherical shape are incorporated into the measured values. Such deviations may result from a prismatic or cylindrical grinding superimposed onto the spherical grinding as well as from the arrangement of a reading portion within the spectacle lens. The off-center placement of the optical axis of the spectacle lens with regard to the geometric axis of the spectacle lens, respectively, the geometric center point of the spectacle frame, also plays a part.
The invention is therefore concerned with providing a method for detecting the bevel course along the edge of the spectacle lens to be machined and for controlling the shape grinding step according to the determined bevel course, whereby the degree of utilization of a CNC-controlled spectacle lens grinding machine should be improved and the precision of the bevel course should be increased, while operation should be simplified.